This invention relates to methods and apparatuses for measuring the average velocity of a flow stream.
In one class of velocity measuring apparatuses, ultrasonic signals are transmitted through the flow stream of a fluid and the reflections received from reflective portions of the fluid are sensed. The Doppler shift in frequency between the transmitted signal and the received signal is used to determine the average velocity of the fluid.
In this class of average-velocity measuring apparatuses, the waveform for the combined Doppler shifts in frequency of reflected ultrasonic sound represents an average velocity of the flow stream because the Doppler frequency shift for each portion of the flow stream is proportional to the velocity of that portion, the amplitude of the sum of the reflected signals for each different frequency shift represents the volume of the fluid flowing with that velocity, and thus, the sum of the received signals is a waveform combining the amount of each different velocity portion of the fluid. Each different velocity portion of the fluid stream contributes to a component of the waveform and its component is proportional to the contribution from each other different velocity portion so that the amplitude for each corresponding frequency shift represents the proportionate amount of fluid flowing with that velocity.
The signals that are incident on reflecting portions of the flow stream near the transmitting transducer or transducers have a higher amplitude than those incident on reflecting portions of the flow stream more remote from the transmitting transducer. The difference in amplitude or intensity is caused by the distribution of energy through a solid angle as it moves from the transmitting transducer to the reflecting portion of the fluid. However, the energy incident on the remote portions impacts on a larger proportion of the fluid at each velocity at more remote distances than at close distances for reflection.
It is believed that this class of average-velocity measuring apparatuses relies on the nature of the flow stream and the intensity of the transmitted signal being such that an approximate compromise can be reached in which the attenuation and reduction intensity with distance is balanced by the increased area from which signals are reflected. This attenuation is caused by the wider distribution of the energy of the transmitted signal and the increased attenuation of the reflected signal over the longer distances. This balance causes the energy transmitted to areas at a distance before being reflected to result in a sensed signal the same as if the entire reflected energy had been reflected from the same plane in the cross-section of the flow stream so that the signal is representative of an average velocity of that cross-section.
Because the received signals mainly represent those sound waves that travel a straight path to the reflective portions of the flow path and are reflected in a straight path to the receiving transducer, the received signals do not include representative amounts of sound waves that are reflected at an obtuse angle such as by glancing off at an angle from a portion of the flow path nor do they include representative amounts of reflected sound waves from certain sides or low portions of the flow path. Thus, the final waveform may actually not include sound waves reflected from the entire cross-section because the transmitted waves miss some portions of the flow stream and some of the reflected waves do not impact directly on the receiving transducer. However, the final waveform must represent the total cross-section.
To cause the final waveform to represent the total cross-section, even though the receiving transducer does not receive a representative amount of sound waves from every portion of the flow path, a representative portion of the flow stream should be selected for measurement of average velocity in this class of average-velocity measuring instrument. This representative portion can be sensed by selecting the angle of the transducers to cut proportional amounts of each velocity of flow.
One prior art velocity measuring system of this class was manufactured and sold by a corporation called Montedoro-Whitney Corporation. That prior art apparatus received different frequency signals in the expected range on a transducer and filtered a set of frequencies which were then weighed and averaged.
This type of measuring apparatus has several disadvantages, such as for example: (1) the range of signals of interest shifts as the velocity of the flow stream shifts, resulting in some inaccuracies due to the selection of a less desirable set of frequencies to be examined; (2) the on-line measurements of a limited number of ranges of frequencies accomplished by that system results in some lack of precision; and (3) because of the lack of precision, an empirically determined velocity coefficient is desirable at most locations to correct the measurement. SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a novel apparatus for measuring the average rate of flow of a liquid in a flow stream.
It is a further object of the invention to provide a novel technique for measuring the average velocity of a stream.
It is a still further object of the invention to provide a novel Doppler shift apparatuses and methods for measuring average velocity that have high resolution.
It is a still further object of the invention to provide a novel Doppler-shift average-velocity meter that is able to directly correct for changes in the turbidity of the stream in a manner independent of variations in the amplitude of the combined reflected ultrasonic vibrations and noise received directly from the transmitted ultrasound.
It is a still further object of the invention to provide a Doppler-shift average-velocity measuring instrument that is capable of providing precision better than 30 percent of the actual average velocity without the need for correcting the received signal with an empirically determined velocity coefficient.
In accordance with the above and further objects of the invention, an average flow rate meter includes an ultrasonic Doppler transmitter and receiver under the control of an automatic range and threshold setting system. The velocity meter transmits sound through a representative section or through the entire cross section of the flow stream and receives a complex signal back which is digitized and analyzed using a fast Fourier transform analyzer.
The resolution of the measurement depends on the number of ranges of frequencies selected for each term of the Fourier transform analyzer across the full range of frequency shifts caused by the range of possible velocities in the flow stream. The expected velocity range is determined in the preferred embodiment and 256 bands of frequencies are selected for positive and negative terms of the Fourier transform analyzer.
To determine the expected velocity range, the input signal is sampled with a high sampling rate such as 11.1 kHz (kilohertz) and a correspondingly high cutoff point for the frequency shift. If the results of the Fourier transform do not provide a high energy distribution in the frequency range being sampled, the sampling rate and cutoff frequency are reduced until a range of frequencies representing a corresponding range of velocities is found that indicates the mid-range of the measured frequency shifts.
The transmitting tranducers and receiving transducers are acoustically shielded from each other in a single housing. This minimizes the amount of noise transmitted directly from the transmitting transducer to the receiving transducer and permits the control of gain in response to changes in the digital, processed signal as determined by the computer when an appropriate cutoff point for the low-pass filters has been found.
To calibrate the average-velocity meter, measurements are made using a model flow path with a known average velocity or an average velocity that can be precisely measured such as by collecting the fluid over a time period and measuring it. The values can be set for measured signals to equal the measured average flow rate.
A threshold amplitude is experimentally set at a level that may cause some received signals having Doppler frequency shifts to not be considered in the final calculation of average velocity because they are represented by low amplitude coefficients in the Fourier transform. This is done until an optimum value is found with a number of terms of the Fourier transform centered around the highest coefficient that provides the best result within the expected range of frequency shifts. This threshold is set for incorporation of the number of reflected signal frequency bands that provides the most constant readout during calibration.
As can be understood from the above description, the average-velocity flowmeter of this invention provides precision in a number of different flow paths and is less likely to have its precision disrupted by changes in the velocity of the stream.